The present invention is directed to a method and apparatus for extrusion blow molding thermoplastic articles, and more particularly to a method and apparatus for extrusion blow molding circumferentially or biaxially oriented monolayer and multilayer polyethylene terephthalate articles.
Currently, blow molded thermoplastic containers of polyethylene terephthalate (PET) are primarily made in one of two ways: two stage injection stretch blow molding or single stage injection stretch blow molding. Known injection stretch blow molding processes generally fit into one of two categories. In the first type of process, a preform is injection molded and then allowed to cool to room temperature. The preform is then stored or sent immediately to a blow molding station where the formed preform is blown to the shape of the desired article. Prior to blow molding, however, the preform has to be heated up to its blow molding temperature. This type of process is disadvantageous in that it is highly energy intensive.
In the second type of injection stretch blow molding process the preform is injection molded and passed, while it is still in a heated condition, to a blow station where it is blown. This type of process requires considerably less energy than the first type of process, however, both processes are disadvantageous in that production is inherently limited because the injection molding step is intermittent. That is, the molten thermoplastic is injected as a "shot" into the preform mold, which is not a continuous operation. Another disadvantage of injection blow molding is the high cost of tooling for the die head and preform mold. A new die and preform mold are required for each different shaped article produced because the preform from which the desired article is blown must meet very specific dimensional requirements in order to form the desired article when blown.
Another commonly used method for producing thermoplastic articles, but not PET articles, is extrusion blow molding. In this type of process the thermoplastic material is continuously extruded in hollow tube form, segments of the pipe are enclosed in a blow mold and the desired article is blown. One material commonly used in such a process is a copolymer of ethylene terephthalate and diethylene glycol, referred to as PETG. This and other copolymers which are capable of being extrusion blow molded oftentimes have limited use due to the fact that they have insufficient moisture impermeability or may not pass migration tests.
Polyethylene terephthalate (PET) resins are excellent molding compounds for making hollow articles such as beverage containers and the like. PET possesses good impact strength and can be formed into clear or colored articles, as desired. In addition, PET has good fatigue resistance and therefore its uses include applications where the product is subjected to fatigue inducing conditions, such as squeeze bottles. It is known to make various PET containers by the injection molding processes described above, however, such processes have the attendant disadvantages mentioned. Additionally, it is known to extrude PET in the form of a pipe and immediately cool the PET pipe below its glass transition temperature (Tg) subsequent to extrusion. The PET pipe is then cut into predetermined tube lengths which can be stored or formed into preforms; in the latter instance, the tubes are heated at both ends and the closed end and finish (threaded area) are formed therein using one of various known methods. The preforms are then heated and blown using known blow molding techniques. Methods such as that described, however, are disadvantageous in that they require several extra steps for fabricating the final product. That is, cooling the pipe, cutting the pipe into predetermined lengths and reheating to form the closed end and finish portions.
In the molten state, and especially at temperatures above about 520.degree. F., PET behaves as a Newtonian fluid; that is, the rate of shear is directly proportional to the shearing force. As a consequence, molten PET has poor melt strength. It has thus heretofore been thought, and recognized in the prior art, U.S. Pat. Nos. 4,188,357 and 4,307,060, that PET containers could not successfully be produced by the process of extruding a soft tube of PET, enclosing the tube in a blow mold and blowing the article. The rationale for this is that at the temperature necessary for extrusion as a tube, PET will not maintain its tubular shape long enough to enclose the tube in a blow mold because of its poor melt strength.
What is needed is a method and apparatus whereby circumferentially or biaxially oriented polyethylene terephthalate articles are formed continuously by extrusion blow molding.